1. Field of the Invention
The present invention is directed to a low leakage, low dropout transistor charging circuit, and more particularly, to a charging circuit which allows efficient battery charging while reducing leakage current from the battery once the battery charger is removed.
2. Discussion of the Prior Art
Many devices, such as wireless telephones, have rechargeable batteries which are charged when needed. FIG. 1 shows a typical charging circuit 10, where a charging or power transistor Q.sub.p is provided between the battery 12 and a charging terminal 14. In particular, the emitter of the power transistor Q.sub.p is connected to the input or charging terminal 14 charging circuit 10, and the collector of the power transistor Q.sub.p is connected to the output 16 of the battery 12. A current limiting resistor R.sub.1 is connected to the base of the power transistor Q.sub.p. Further, a resistor R.sub.2 is connected between the charging terminal 14 (or emitter) and base of the power transistor Q.sub.p. In addition, a pull down resistor R.sub.p is connected between the charging terminal 14 and ground.
The battery 12 provides power to the wireless telephone which is represented by a load resistor R.sub.L connected to the battery output 16. For charging the battery 12, a charge source 18 is connected to the charging terminal 14 and the power transistor Q.sub.p is turned ON in response to a signal
from a controller 20, which is a digital signal processor (DSP) or microprocessor, for example.
In particular, the controller 20 is connected to the base of a control transistor Q.sub.c through a control base resistor R.sub.3. Illustratively, the control transistor Q.sub.c is an NPN bipolar transistor and the power transistor Q.sub.p is a PNP bipolar transistor. For proper biasing, another resistor R.sub.4 is connected between the base of the control transistor Q.sub.c and ground. The collector of the control transistor Q.sub.c is connected to the base of the power transistor Q.sub.p through the current limiting resistor R.sub.1. Further, the emitter of the control transistor Q.sub.c is connected to ground.
In the active or charge mode where the charger 18 is connected to the charging terminal 14 for charging the battery 12, e.g., when the wireless telephone is placed in its cradle for charging, the micro-controller 20 must provide a signal to turn ON the control transistor Q.sub.c. Thus, it is necessary to detect when the charger 18 is connected to the charging terminal 14.
The PNP power transistor Q.sub.p turns ON when its emitter-base junction is forward biased, i.e, when its emitter voltage is greater than its base voltage. This condition occurs when the charger 18 is connected to the charging terminal 14 to provide a positive charging voltage, and when the base of the power transistor Q.sub.p is grounded through the current limiting resistor R.sub.1 and the ON control transistor Q.sub.c. For example, the power transistor Q.sub.p in ON when the emitter voltage is 0.5.about.0.7 volts more than the base voltage, where 0.5.about.0.7 volts is the voltage drop across the emitter to base junction. The ON power transistor Q.sub.p allows a charging current I.sub.c to pass from the charger 18 to the battery 12 for charging thereof.
One problem with the conventional charging circuit 10 is back current leakage from the battery output 16 to ground, through the collector-base (C-B) junction of the power transistor Q.sub.p when the charger 18 is not connected to the charging terminal 14, referred to as the idle mode.
In the idle mode, the C-B junction of the power transistor Q.sub.p is forward biased by the battery voltage. The C-B or leakage current I.sub.L flows through the current limiting resistor R.sub.1 if the control transistor Q.sub.c, is ON and saturated. In addition, the C-B or leakage current I.sub.L also flows through resistor R.sub.2, and the pull-down resistor R.sub.p to the ground. The C-B forward bias current, also shown in FIG. 1 as the leakage current I.sub.L, brings the power transistor Q.sub.p to the saturation mode.
In the saturation mode, the voltage drop across C-E of the power transistor Q.sub.p becomes approximately 0.1 V, and thus the battery voltage is effectively applied to the charging terminal 14. Therefore, the leakage current I.sub.L is defined by the current through the pull-down resistor R.sub.p. The pull-down resistor R.sub.p is provided to pull down the voltage of the charging terminal 14 when the charger 18 is disconnected therefrom.
If the control transistor Q.sub.c is ON, then in addition to the current leakage through the pull-down resistor R.sub.p, there is further leakage current through the C-E of ON control transistor Q.sub.c to ground. Due to the need to have a relatively small current limiting resistor R.sub.1, the additional leakage current through the current limiting resistor R.sub.1, when the control transistor Q.sub.c is ON, is relatively large, thus accelerating the premature discharge of the battery 12.
In addition to the premature battery discharge, another disadvantage of the conventional charging circuit 10 is the inability to detect whether the charger 18 is connected to the charging terminal 14. Knowledge of whether the charger 18 is connected to the battery is needed by the controller 20 for proper operations and power management.
Detection of a low voltage at the charging terminal 14 indicates the charger 18 is disconnected therefrom. That is, the presence or absence of the charger 18 is determined by measuring the voltage at the charging terminal 14. A high voltage indicates that the charger 18 is connected to the charging terminal 14, and a low voltage indicates that the charger 18 is disconnected from the charging terminal 14, e.g., the phone R.sub.L is not on its charging cradle 18. However, because of the saturation effect of the power transistor Q.sub.p, the emitter voltage of the power transistor Q.sub.p is approximately equal to its collector voltage, namely, the battery voltage. Due to the saturation of the power transistor Q.sub.p resulting in nearly equal emitter and collector voltages, where the collector voltage is the battery voltage, the voltage at the charging terminal 14 remains high even when the charger 18 is disconnected. Thus, in the conventional charging circuit 10, it cannot be determined whether the charger 18 is connected to or disconnected from the charging terminal 14.
FIG. 2 shows a second conventional charging circuit 50 where a diode D.sub.1 is connected in series between the collector of the power transistor Q.sub.p and the battery output 16. The diode D.sub.1 prevents flow of the leakage current, shown in FIG. 1 as I.sub.L, from the battery 12 to ground through the C-B junction of the power transistor Q.sub.p. The second conventional charging circuit 50 is identical to the conventional charging circuit 10 shown in FIG. 1 except for the diode D.sub.1. The leakage current I.sub.L (FIG. 1) from the battery 12 to the diode D.sub.1 does not flow because the diode D.sub.1 is reversed biased, and thus OFF, when the charger 18 is disconnected from the charging terminal 14.
When the charger 18 is connected to the charging terminal 14, the diode D.sub.1 allows battery charging. This is due to the diode D.sub.1 being forward biased, and thus ON, since typically the charger voltage is at least 0.8 volts greater than the battery voltage, where 0.8 volts is the voltage drop across the diode D.sub.1 and the E-C voltage drop of the saturated power transistor Q.sub.p is approximately 0.15 V. The ON diode D.sub.1 allows flow of the charging current I.sub.c from the charger 18 to the battery 12 for charging thereof.
Further, when the charger 18 is disconnected from the charging terminal 14, the voltage at the charging terminal 14 is zero due to the ground connection through the pull 25 down resistor R.sub.p. The voltage at the charging terminal 14 is reduced to zero because the charging terminal 14 is open, and there is no leakage current I.sub.L (FIG. 1) flowing through the collector to base (C-B) junction of the power transistor Q.sub.p. The absence of the leakage current I.sub.L prevents saturation of the power transistor Q.sub.p which prevents nearly shorting its emitter to its collector. The open power transistor Q.sub.p prevents connection of its emitter to the battery 12, resulting in a zero emitter voltage due to the pull-down resistor R.sub.p and the open emitter of the OFF power transistor Q.sub.p.
Although adding the diode D.sub.1 prevents premature battery discharge and allows detection of the charger 18 being connected to the charging terminal 14, the diode D.sub.1 has several disadvantages. In particular, the diode D.sub.1 is expensive and large, thus increasing cost, consuming valuable real estate, and preventing miniaturization. Further, the diode D.sub.1 has a voltage drop of approximately 0.8 volts, thus increasing the required charging voltage by approximately 0.8 volts and consuming high power, such as approximately 0.56 (0.8 v.times.0.7 A) watts for a charging current I.sub.c of approximately 700 mA. The increased power dissipation generates unwanted heat which is often detrimental to the proper operations of devices connected to the output 16 of the conventional charging circuit 50, such as a wireless telephone which includes temperature sensitive circuits or components like clock oscillators.
Accordingly, there is a need for a charging circuit which prevents leakage current, allows detection of connection or disconnection of the charger from the battery, and minimizes power consumption and heat generation.